In a conventional image forming apparatus employing an electrophotographic printing method, a charging device employing a corona discharge system has been frequently used as: a charging device for charging a photoreceptor; a transfer device for electrostatically transferring, to recording paper or the like, a toner image which is formed on the photoreceptor or the like; and a separation device for separating the recording paper or the like which is electrostatically in contact with the photoreceptor or the like.
Such a charging device employing the corona discharge system generally includes a shield case having an opening section which faces a charge receiving materials such as the photoreceptor and the recording paper, and a discharge electrode of a line or saw-tooth shape which discharge electrode is tensed in the shield case. Examples of this charging device include: (i) a corotron charger which uniformly charges charge receiving materials by applying a high voltage to the discharge electrode so as to cause corona discharge, and (ii) a scorotron charger (refer to Patent Literature 1) which uniformly charges charge receiving materials by applying a desired voltage to a grid electrode provided between a discharge electrode and the respective charge receiving materials.
Patent Literatures 2 and 3 disclose that the charging device employing the corona discharge system is used in a charging device for charging before a transfer in which a toner image is charged before the toner image is transferred to transfer mediums such as the intermediate transfer member and the recording paper. According to techniques disclosed in Patent Documents 2 and 3, even if a charge amount is not uniform in a toner image formed on an image bearing member, the charge amount of the toner image is uniformized before the toner image is transferred. This allows (i) suppression of a decrease in transfer margin required for transferring a toner image and (ii) a stable transfer of the toner image to a transfer medium.
Note however that the conventional charging device as mentioned above has the following problems. First, the charging device requires not only the discharge electrode but also the shield case, the grid electrode, and the like. Further, it is necessary to secure a given distance (10 mm) between the discharge electrode and the respective charge receiving materials. This requires a large space for providing the charging device. Generally, a developing device, a first transfer device, and the like are provided around a first transfer section, and the photoreceptor, a second transfer device, and the like are provided in front of a second transfer section. Therefore, a space is small for providing the charging device for charging before a transfer. This causes a problem that it is difficult to make a layout in the conventional charging device employing the corona discharge system.
On the other hand, in recent years, a charging device employing a contact charging system has been used as a charging device for charging a photoreceptor itself. In the contact charging system, a conductive roller or a conductive brush carries out contact electrification. However, it is difficult to carry out charging without disordering the toner image by the contact charging system. Further, in case of employing the contact charging system, a contact with the photoreceptor or spark discharge generated in a micro air gap is highly likely to accelerate a deterioration in photoreceptor. This is an obstacle to a life extension of the photoreceptor and ultimately to a reduction in running cost. In view of this, a non-contact corona discharge system is to be used as the charging device for charging before a transfer.
In order to address the problems of the conventional charging device, for example, Patent Literature 4 discloses a charging device which includes an ion generating device (a creeping discharge device) employing a charging system (hereinafter, a charging system of this type is referred to as a creeping discharge system) in which: a discharge electrode having pointed protrusions on an outer periphery of the discharge electrode and an induction electrode are provided so as to sandwich a dielectric body between the discharge electrode and the induction electrode, and an alternating high-voltage is applied across the discharge electrode and the induction electrode so as to generate ions.
The charging device employing this creeping discharge system is small in size because the charging device has no shield case and no grid electrode. Moreover, it is easy to clean the charging device because the charging device has a flat discharging surface. Therefore, the charging device excels in maintenance. Furthermore, the charging device is not in contact with charge receiving materials and does not directly discharge surfaces of the respective charge receiving materials. Therefore, it is possible to minimize a deterioration in each of the charge receiving materials.
Note here that the ion generating device (creeping discharge device) tends to have a weaker discharge characteristic under a high humidity environment. In order to avoid this, for example, Patent Literatures 5, 6, 7, and 8 disclose techniques for improving discharging performance by providing the ion generating device with a heater member and by heating the device to remove absorption moisture on a discharge area. In particular, Patent Literatures 6 and 8 disclose a technique which also functions as a heater by applying a voltage to an induction electrode to generate Joule heat. According to the technique disclosed in Patent Document 6, it is possible to make the ion generating device more compact and lower in cost as compared with the technique of independently providing an additional heater device.